Update the DOC about the easier way to add libs via single package.json only

[PeterDaveHello]

Since the repo is too fat, we actually support submit PR to add a lib with only one single package.json without real files, so the users don't need to clone the repo locally, just do the commit and PR on GitHub.

Example:
#7149 by @PeterDaveHello
#7725 by @pakastin
cc #3941

[pakastin]

I actually started cloning the repo first, but canceled it and searched for a better way 😂

Using a single package.json was really simple. 👍

[pakastin]

..I ended up using just the Github's web interface — really straight forward.

[PeterDaveHello]

@pakastin I wonder if you would like to improve the document to talk about this part 😄

[PeterDaveHello]

@cdnjs/intern2 @cdnjs/intern everyone who is interested in this issue please just take it!

[maruilian11]

https://github.com/cdnjs/cdnjs/blob/master/CONTRIBUTING.md#e-adding-a-new-library-by-a-single-packagejson